Sphere is a solid figure shaped like a ball or globe. The term sphere comes from the Greek word sphaira, meaning ball. In geometry, mathematicians define a sphere as a set of all points in space a certain distance from a fixed point called the center. This means that a sphere is a solid figure bounded by a single surface. The surface does not have any edges or boundaries, and each of the points on the surface is the same distance from the center.
The radius of a sphere is the distance from the center to the surface. The radius also can be defined as any straight line drawn from the center to the surface. The diameter of a sphere is twice the radius. It also is any straight line drawn through the center whose ends stop at the surface. A secant is any line that cuts through a sphere. A chord is any line that joins two points on the sphere.
If a plane (flat surface) passes through the center of a sphere, the plane produces a great circle of the sphere. The radius and diameter of a great circle are the same as the radius and diameter of the sphere. A great circle cuts a sphere in half. Each half is called a hemisphere.
The area of the surface of a sphere can be found by using the following formula: 
In the formula, the letter r stands for the radius. An approximate value for π is 3.1416. The surface computed with this formula will appear in square units, such as square inches or square centimeters.
The volume of a sphere can be calculated with the formula: 
The letter r again represents the radius of the sphere. The volume computed with this formula will appear in cubic units, such as cubic inches or cubic centimeters.
